1. Field of the Invention
The invention relates to a hold-down device for Jacquard-controlled looms.
2. Discussion of the Prior Art
With arrangements of this kind the individual warp threads are moved by means of cord assemblies between an upper and a lower position to form the shed. By means of a pull-down device exerting spring force the warp threads or the lower end of the cord assembly are prestressed towards the frame of the machine in the lower shed position. Lifting the warp thread and the cord assembly into the upper shed position is achieved by control of the hooks of a Jacquard machine according to a pattern, the lower end of the hooks of the Jacquard machine being directly, and usually releasably, connected to the upper end of the cord assembly by means of a hook cord. In the lower shed position the respective hook of the Jacquard machine is supported on a bottom board, and in the upper shed position, when controlled, the hook is held by means of a suitable arresting device. A large number of systems for controlling the hooks of a Jacquard machine according to a pattern are known and used commercially, for example mechanical and electromechanical systems.
Generally the lower ends of all the hooks of a Jacquard machine have a different geometric spatial arrangement than the points of engagement of the cord assemblies with the warp threads, so that guides are necessary for the cord assemblies, which result in different amounts of friction. Furthermore it is usual to lift or to lower a plurality of warp threads by means of one hook, i.e. to provide a plurality of hook cords and cord assemblies. When weaving with a pattern repeat in this way up to 10 cord assemblies can be controlled by means of one hook, so that a correspondingly greater force is exerted on the hook by the plurality of pull-down devices. The number of cord assemblies arranged in a pattern repeat can alter from one piece of weaving to another. There are thus different loads depending on the job. If, however, the force exerted by the pull-down device is too great, wear is high. The desired harness life, the operating time of the loom with a Jacquard machine, is thereby reduced.
If on the other hand the force exerted by the pull-down device is too low operational difficulties can result. These disadvantages become greater at faster operating speeds since at a higher speed of rotation of the drive for the Jacquard machine the hooks are moved at greater speeds and also have to be returned by the spring force.
The difficulty therefore lies in combining the requirements of the weaving process and the mechanical requirements so as to keep the harness life as high as possible.
If the force exerted on the hook was too low, craftsmen sometimes also stretched a spring between the hook and the frame of the machine. For constructional reasons, and because of the inaccessibility of the numerous hooks in a Jacquard machine, this was only done occasionally, and furthermore was only possible with hooks at the outside.
Different loads on the weaving machine can occur depending on the application for which the weaving is performed. A requirement of weaving technology lies in the fact that the spring force of the pull-down device must always be sufficiently high in order to prevent operational disturbances, such that the force of the pull-down device must be sufficiently high in order to enable the machine to operate efficiently at any rate that the hook is pulled back. On the other hand, a mechanical requirement for the machine must be met, in order to prevent excessive wear which may occur in the case of excessive exertion of force. This disadvantage is particularly serious if the machine is operated with a pattern repeat, that is to say, if more than one cord assembly is controlled by means of a single hook. In addition, the Jacquard loom shall be designed such that by means of retrofitting the machine during standstill, a changing number of cord assemblies may be controllable by means of one hook, thus efficient operation of the machine is insured while wear is reduced even at high speeds.